A biosensor of the aforementioned kind is known from the WO 2010/150167 A1. In the known biosensor, a plurality of lenslets are arranged in an array for focusing an incident parallel light beam that is generated by frustrated total internal reflection onto the sensor plane of an image sensor. Homogeneous images can be achieved this way that are not distorted by field curvature. However, detection NA is very low and the images obtained have a low resolution.